MRI Apparatus with RF corrected each TR interval

Abstract

An MRI apparatus includes, a generating unit configured to generate radio frequency pulses applied in a pulse sequence; a sequence control unit configured to apply a radio frequency pulse related to acquisition of an image signal and a corrective radio frequency pulse during execution of one TR of a pulse sequence; and a calculation unit configured to measure the corrective radio frequency pulse and calculate a correction value for the radio frequency pulse. Based on the correction value, the generating unit corrects a radio frequency pulse related to acquisition of an image signal to be applied during a following TR later than a TR during which the corrective radio frequency pulse is measured.

What is claimed is: 
     
       1. An MRI apparatus comprising:
 a generating unit configured to generate radio frequency (RF) pulses according to a pulse sequence, the pulse sequence including a repeating series for each of plural repetition time (TR) intervals; 
 a sequence control unit configured to apply an imaging RF pulse acquiring an imaging signal and a corrective RF pulse for correction following the imaging RF pulse during each TR interval of the pulse sequence thereby applying plural sets of RF pulses, each set including an imaging RF pulse and a corrective RF pulse, during each of the plural TR intervals of the pulse sequence; 
 a wave detector configured to detect the corrective RF pulse during each of the plural TR intervals; and 
 a calculation unit configured to calculate a correction value based on the detected corrective RF pulse during each TR interval, 
 wherein the generating unit is configured to correct an imaging RF pulse to be applied during a second and subsequent TR intervals of the plural TR intervals based on the correction value detected during earlier TR intervals in the pulse sequence. 
 
     
     
       2. The MRI apparatus according to  claim 1 , wherein the calculation unit calculates the correction value based on a corrective RF pulse applied when a spoiler gradient magnetic field is applied. 
     
     
       3. The MRI apparatus according to  claim 1 , wherein the calculation unit calculates the correction value based on a corrective RF pulse applied at a site outside an imaging region. 
     
     
       4. The MRI apparatus according to  claim 1 , wherein the corrective RF pulse applied to calculate the correction value is a part of an imaging sequence. 
     
     
       5. The MRI apparatus according to  claim 1 , wherein the corrective RF pulse is measured by a pickup coil, the pickup coil being a receiving coil configured to receive imaging RF pulses. 
     
     
       6. The MRI apparatus according to  claim 1 , wherein the corrective RF pulse is measured via a directional coupler configured to input the imaging RF pulses outputted from the generating unit. 
     
     
       7. The MRI apparatus according to  claim 4  wherein a triangular wave is used as the corrective RF pulse applied to calculate the correction value. 
     
     
       8. The MRI apparatus according to  claim 4 , wherein a sinc wave is used as the corrective RF pulse applied to calculate the correction value. 
     
     
       9. The MRI apparatus according to  claim 1 , wherein the corrective RF pulse applied to calculate the correction value is outputted in an amplitude that includes a maximum value and minimum value of the imaging RF pulse. 
     
     
       10. The MRI apparatus according to  claim 1 , wherein the calculating unit calculates the correction value based on the corrective RF pulses which are different in amplitude among different TRs. 
     
     
       11. The MRI apparatus according to  claim 1 , wherein:
 the calculation unit calculates a correction value each time the corrective RF pulse is outputted; and 
 according to the correction value, the generating unit generates a corrected imaging RF pulse by correcting the imaging RF pulse outputted in a TR interval subsequent to the TR interval in which the correction value is calculated. 
 
     
     
       12. The MRI apparatus according to  claim 1 , wherein:
 the calculation unit calculates a correction value each time the corrective RF pulse is outputted a predetermined number of times; and 
 the generating unit generates a corrected imaging RF pulse by correcting output of the imaging RF pulse according to timing of calculating the correction value. 
 
     
     
       13. A magnetic resonance imaging (MRI) apparatus having provisions for limiting radio frequency (RF) heating of an imaged object to a safe specific absorption ratio (SAR) value, said apparatus comprising:
 an RF generator connected to feed an imaging RF pulse acquiring an imaging signal and a corrective RF pulse for correction to at least one RF coil during execution of an MRI pulse sequence of plural repetition time (TR) intervals, the imaging RF pulse and the corrective RF pulse following the imaging RF pulse being generated during each TR interval of said plural TR intervals, thereby applying plural sets of RF pulses, each set including an imaging RF pulse and a corrective RF pulse, during each of the plural TR intervals of the pulse sequence, the pulse sequence including a repeating series for each of said plural TR intervals; 
 an RF signal amplitude detector positioned so as to sample and measure plural amplitudes of corrective RF outputs generated by the RF generator as a function of control inputs thereto during said MRI pulse sequence; and 
 SAR control circuitry connected to said RF generator and said RF signal amplitude detector, said SAR control circuitry being configured to calculate an RF pulse amplitude correction value based on a detected corrective RF pulse during a first TR interval of said plural TR intervals and which controls the RF generator in accordance with previously measured input versus output RF generator responses so as to thereafter correct the amplitude of an imaging RF pulse to be applied during a second TR interval of said plural TR intervals following the first TR interval in said MRI pulse sequence thereby more accurately and safely controlling the SAR. 
 
     
     
       14. A magnetic resonance imaging (MRI) method for limiting radio frequency (RF) heating of an imaged object to a safe specific absorption ratio (SAR) value, said method comprising:
 generating and transmitting an imaging RF pulse acquiring an imaging signal and a corrective RF pulse for correction to at least one RF coil during execution of an MRI pulse sequence of plural repetition time (TR) intervals, the imaging RF pulse and the corrective RF pulse following the imaging RF pulse being generated and transmitted during each TR interval of said plural TR intervals, thereby applying plural sets of RF pulses, each set including an imaging RF pulse and a corrective RF pulse, during each of the plural TR intervals of the pulse sequence, the pulse sequence including a repeating series for each of said plural TR intervals; 
 sampling and measuring plural amplitudes of corrective RF outputs generated by an RF generator as a function of control inputs thereto during said MRI pulse sequence; and 
 calculating an RF pulse amplitude correction value based on a detected corrective RF pulse during a first TR interval of said plural TR intervals and using it to control the RF generator in accordance with previously measured input versus output RF generator responses so as to thereafter control the amplitude of an imaging RF pulse to be applied during a second TR interval of said plural TR intervals following the first TR interval in said MRI pulse sequence thereby more accurately and safely controlling the SAR.